Condensational growth systems have been used to enlarge sub micrometer sized aerosolized particles to form droplets. These aerosolized particles may be airborne, or carried by another process gas such as nitrogen, but are defined as particles of condensed matter (liquid or solid) suspended in a gas that is too small to settle by gravity in the time scales of interest. Often the aerosolized particles of interest are less than a few micrometers in diameter. To facilitate their measurement, these particles may be grown through condensation to form larger droplets that can be detected or manipulated much more easily than the original particle. For example, the droplets can be detected optically, captured inertially, or focused aerodynamically.
For small particles, the initiation of condensational growth is created by exposure of the particle to a region of vapor super-saturation, defined as a region in which the vapor pressure of the condensing vapor is higher than its saturation value over a flat surface. These supersaturated conditions are used because the equilibrium vapor pressure above the curved surface of an ultrafine particle is higher than over a flat surface of the same chemical composition. This is due to the energy associated with the surface tension, a phenomenon described by the Kelvin relation. Water condensation onto small particles requires relative humidity values in excess of 100%. Roughly speaking, the relative humidity needed to activate the condensational growth of particles below 6 nm in diameter is in the range of 140% and larger. The exact value of the super-saturation required also depends on particle chemistry, which for soluble salts is described by the Kohler relations.
It is not possible to have super-saturated conditions at walls of the container carrying the flow, as any excess water vapor will simply deposit on the walls. However, it is possible to create non-equilibrium conditions within the core of the flow, thereby temporarily providing supersaturated conditions. Methods of achieving this include (1) adiabatic expansion of a saturated flow, (2) the rapid (generally turbulent) mixing of flows of differing temperatures, and (3) laminar flow diffusion wherein a cooler flow is introduced into a warm, wet-walled tube.
With each of these methods the time that the flow experiences supersaturated conditions is limited. For example, with the laminar flow methods the supersaturation profiles exhibit a sharp maximum, and then decay. As most of the condensational growth occurs in this peak supersaturation region, this limited time also limits the size of the droplet that is formed. This is especially an issue when operating at low supersaturations, such as is typical of clouds. Also, the activation of condensational growth can be kinetically limited, such that this short supersaturation is insufficient to initiate condensational growth on those particles that are hydrophobic, i.e. that have an inherent low probability for water molecules to adhere to their surface.